The present invention relates to an f..theta. lens system for use in a scanning optics system. More particularly, the present invention relates to an achromatized telecentric f..theta. lens system of a four-unit composition.
Various printers, plotters, facsimiles and other equipment that employ a laser beam are known. In the scanning optics of such apparatuses, a laser beam is scanned with constantly a rotating mirror such as a polygonal mirror in such a manner that the angular scanning velocity is proportional to the speed of movement of an image. In such optics, an f..theta. lens system is commonly employed that satisfies the relation y.perspectiveto.f..theta., in which f denotes the focal length of the lens, .theta. is the angle at which the beam enters the lens, and y is the height of a real image.
Many f..theta. lens systems have been known, but since all are intended to be used with laser light having a single wavelength, compensation for chromatic aberration has not been included in their design considerations.
An achromatic f..theta. lens, however, is desirable for compatibility with a laser that operates in more than one longitudinal mode. Such a multi-mode operating laser produces a higher energy-output than a single-mode laser and offers advantages such as a higher imaging speed in laser photoplotters. Therefore, a single-mode laser that produces a certain output power can be replaced by a multi-mode laser of a smaller size. Taking an argon laser operating at a single wavelength .lambda. of 488 nm as an example, approximately three times as high an output can be produced from a multi-mode argon laser operating over a wavelength range of 488 to 514.5 nm.
A telecentric optical system is also known in the art. In this system, the principal ray emanating from an off-axial object point extends at an angle substantially perpendicular to the image plane. A major advantage of a telecentric optical system is that, even if defocusing occurs when the imaging plane is slightly displaced in the direction of the optical axis or on account of the presence of corrugation in the imaging surface, a predetermined relationship between the angle of incidence and the height of the image is retained to ensure a comparatively high dimensional accuracy. Telecentric f..theta. lens systems of the above type have been proposed, for example, in Unexamined Published Japanese patent application No. 195211/1984.